1. Field of the Invention
The present invention relates in general to a fluid-filled elastic mount which exhibits a vibration damping or isolating effect based on flows or resonance of a non-compressible fluid such as water contained therein, and more particularly to such a mount which has three orifice passages tuned to respective different frequency bands of input vibrations and exhibits an excellent vibration damping or isolating effect with respect to the input vibrations over a wide frequency range, based on the flows of the non-compressible fluid through these orifice passages.
2. Discussion of the Related Art
As one type of a vibration damping elastic bushing or mount interposed between two members of a vibration system, there is known a fluid-filled cylindrical elastic mount comprising: a metal shaft; an outer sleeve disposed radially outwardly of the metal shaft with a predetermined radial spacing therebetween, such that these two members are co-axial or eccentric with respect to each other; an elastic body interposed between the metal shaft and the outer sleeve for elastically connecting the metal shaft and the outer sleeve, and partially defining a pressure-receiving chamber; a flexible diaphragm partially defining an equilibrium chamber opposed to the pressure-receiving chamber in the diametrical direction of the mount; and means for defining an orifice passage connecting between the pressure-receiving chamber and the equilibrium chamber. The pressure-receiving and equilibrium chambers are filled with a non-compressible fluid. Upon application of vibrations to the mount, the pressure of the fluid in the pressure receiving chamber changes due to deformation of the elastic body, and the volume of the equilibrium chamber changes due to deformation of the flexible diaphragm. The fluid-filled cylindrical elastic mount of this type is capable of exhibiting an excellent vibration damping or isolating effect based on the flows or resonance of the fluid, which is forced to flow through the orifice passage upon application of the vibrations between the metal shaft and the outer sleeve, and is preferably usable as an engine mount for a motor vehicle, for example.
Generally, the above indicated vibration damping or isolating effect of the mount based on the flows or resonance of the fluid is exhibited with respect to only the particular input vibrations over a limited frequency range to which the orifice passage is tuned. However, some engine mounts for the vehicle are required to damp or isolate the input vibrations over a wide frequency range, since the frequency of the input vibrations to be damped or isolated changes depending upon the driving condition of the vehicle. For instance, the engine mount is required to exhibit a high vibration damping effect with respect to low-frequency vibrations of 10 Hz, such as an engine shake, and to exhibit a high vibration isolating effect owing to a low spring constant with respect to medium-frequency vibrations of 20 Hz-40 Hz, such as an engine idling vibration and high-frequency vibrations of 50 Hz-80 Hz such as a booming noise and a high harmonics of the engine idling vibration.
In view of the above indicated requirements, there is proposed a mount structure comprising two orifice passages independent of each other, and two equilibrium chambers independent of each other and connected to the pressure receiving chamber through the respective orifice passages, as disclosed in JP-B-7-99186, JP-A-4-95631 and JPU-6-22641. Based on the resonance of the fluid flowing through the two orifice passages, this mount structure can exhibit a vibration damping or isolating effect with respect to the input vibrations over two different frequency bands to which the two orifice passages are respectively tuned. However, even in the mount structure constructed as described above, the intended vibration damping or isolating operation is effective only to the input vibrations over the two different frequency bands to which the two orifice passages are respectively tuned. Accordingly, there has been a demand for an engine mount which exhibits an excellent vibration damping or isolating effect based on the flows or the resonance of the fluid with respect to the input vibrations over a sufficiently wide frequency range.
It is considered possible to provide the desired engine mount by simply adding one more equilibrium chamber and one more orifice passage to the mount structure disclosed in JP-B-7-99186 and the other publications identified above. Namely, the engine mount includes three independent equilibrium chambers and three independent orifice passages connected to the respective equilibrium chambers, so that the engine mount exhibits the vibration damping or isolating effect with respect to the input vibrations over a wider frequency range, based on the flows of the fluid through the three orifice passages. However, this engine mount does not exhibit the sufficient vibration damping or isolating effect based on the flows of the fluid through the orifice passages, unless the equilibrium chambers of the engine mount permit sufficient amounts of change in their volumes. In this respect, there is a technical difficulty of arranging the three equilibrium chambers in the interior space of the mount, which is limited by the size of the mount, while assuring that the three equilibrium chambers have the desired volumes. Thus, a practically satisfactory mount structure having three equilibrium chambers has not still been developed.